One of my common goldfish is bottom sitting. They recently had parasites and I just finished treatment (API General Cure) I did a water change yesterday and ammonia and nitrite are both 0. I added new carbon. What is causing this? Im guessing from the stress that the parasites put on it. It's also been thundering pretty loud where I live so maybe the noise is stressing him out? He immediately stops and swims around when I'm by the tank.